Hgc levels not rising with triplets

Hi guys, havnt posted in a while and just need some advice/support 

I recently found out I was pregnant after having a miscarriage in December. I had to have an early scan last Wednesday since I was having bad cramping, on my scan tbey found 3 sacs and they took it as an early pregnancy, as they couldn't see anything inside the sacs, (they estimated me at arojnd 5 weeks) They took my bloods and told me to come back Today (Monday) for more bloods to make sure they have doubled, My HCG levels on wednesday were nearly 39000 but today they have only risen another 280 in 5 days, I have anothe scan Wednesday to determine what is happening, I'm At a complete loss with myself, weird but has anyone ever heard of this? I was quite shocked when she said their is a possibility triplets would happen, sorry for rant and long post xxx